[Mpi-forum] cancel in standard

[Jeff Hammond]

What's the harm in letting a message that the user clearly does not
care about sit in a queue/buffer forever?  I consider this an
acceptable consequence of a really stupid feature.

Jeff

On Wed, Jun 3, 2015 at 9:50 PM, William Gropp <wgropp at illinois.edu> wrote:
> Pavan is correct about the intent, and we could make this more precise.  The basic requirement is that a message is *guaranteed* to either be canceled (on send) or received.  Sitting in a queue or buffer somewhere does not count.
>
> Bill
>
> On Jun 3, 2015, at 7:19 PM, Balaji, Pavan <balaji at anl.gov> wrote:
>
>>
>> [Cc'ed the Forum]
>>
>> This wording has been debated many times at the Forum.  Specifically, the "completes normally" and "received at the destination" pieces are ambiguous.  For instance, if the data is copied into a bounce buffer, the send would complete normally, but that does not mean that there is a receive posted for that data.  In this case, it would be the user's responsibility to post a corresponding receive.  If the MPI implementation does not have a bounce buffer (e.g., for large messages), "received at the destination" could mean into an unexpected buffer, not necessarily a user buffer.
>>
>> The consensus from MPI-1 veterans (Marc, Bill, etc.) during the last discussion was that if the application does not post a receive, then it is guaranteed that the send should be cancelable.  At least that was the intent.  The wording, of course, is too vague for such a definition.
>>
>> You could try to weaken that interpretation and say that if the MPI implementation is not able to cancel a send operation, then the user is responsible for posting a matching receive to receive that data.  Irrespective of which interpretation we go with, I think it is important to clarify it in the standard.  Perhaps an MPI-3.1 errata or MPI-3.2 change instead of waiting for MPI-4?  Do you want to create a ticket?
>>
>>  -- Pavan
>>
>>
>>
>>
>> On 6/3/15, 7:05 PM, "Sur, Sayantan" <sayantan.sur at intel.com> wrote:
>>
>>> What lines in the standard support the interpretation that “a message that
>>> will not be successfully matched, must be cancelable”?
>>>
>>> This is what I found, and it doesn’t support the stricter interpretation.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> "If a
>>> send is marked for cancellation, then it must be the case that either the
>>> send completes
>>> normally, in which case the message sent was received at the destination
>>> process, or that
>>> the send is successfully cancelled, in which case no part of the message
>>> was received at the
>>> destination. Then, any matching receive has to be satisfied by another
>>> send."
>>>
>>>
>>>
>>>
>>>
>>>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/